1. Field of the Invention
This invention relates to a stabilized, photosensitive diazo composition and to a pre-sensitized lithographic plate using the same.
2. Description of the Prior Art
Various diazonium compounds, of which a diazo resin is the most common, represented by the condensate of p-diazodiphenylamine with formaldehyde, are used as the photosensitive material in pre-sensitized lithographic plates. A pre-sensitized plate can be prepared by coating such a diazo resin on a suitable support such as paper, a synthetic resin film or a metal, and when the plate is exposed to actinic radiation through a transparent negative original, the diazo resin at the exposed area decomposes and is rendered insoluble. On the other hand, the non-exposed areas can be removed using water so as to uncover the underlying support surface. When a support which has been treated to render the surface hydrophilic is used, development uncovers such a hydrophilic surface at the non-irradiated areas. Thus, when the developed plate is placed in an offset printing machine, these hydrophilic areas are receptive to water and repel ink, while the decomposed diazo resin which remains is oleophilic and is receptive to ink and repels water. In other words, such a photosensitive material enables a positive plate to be produced from a negative original.
Various compositions for the photosensitive layer of this kind of pre-sensitized lithographic material can be divided into two types; one type without any binder, i.e., comprising solely a diazo resin such as is disclosed in, for example, U.S. Pat. No. 2,714,066, and the other type comprising a mixture of a diazo resin with a binder such as is disclosed in, for example, U.S. Pat. No. 2,826,501 and British Pat. No. 1,074,392. In the case of the former type, the coating thickness of the photosensitive layer is small, usually corresponding to a coating weight of from about 20 to about 100 mg/m.sup.2 due to the lack of binder, and thus this results in the image obtained having insufficient abrasion resistance. Thus, reinforcement of such an image which exhibits poor printing performance by itself is necessary. Usually an emulsified lacquer is applied onto the plate during or subsequent to development whereby the resinous ingredient in the lacquer emulsion adheres selectively to the image areas. By such a treatment, the image becomes resistant to abrasion or stripping actions occurring during printing, and therefore, a large number of prints can be produced therefrom. The lacquer used for such a purpose must have superior abrasion resistance, have a high affinity to the image area, and not tend to form scum or redeposit. To meet these requirements, many approaches have been made including the techniques disclosed in, for example, U.S. Pat. No. 2,754,279. An essential problem in this approach is, however, that the ultimate user of the printing material has to carry out this operation of lacquer coating, and as a result, the performance of the printing plate thus treated importantly depends on the skill with which this operation is performed. This means that the operator must always have a sufficiently high level of expertise. To overcome this difficulty an improvement has been developed in which a photosensitive coating is used comprising a photosensitive compound and a highly abrasion-resistant resin mixed therewith which can be cross-linked with the photo-decomposed product from the diazonium compound (e.g., as disclosed in U.S. Pat. No. 2,826,501 and British Pat. No. 1,074,392). In other words, a resinous ingredient corresponding to that contained in the lacquer conventionally employed is blended with a diazo resin and the mixture is coated in an increased coating thickness (about 0.5 to about 5.0 g/m.sup.2); at development unnecessary portions, i.e., unhardened areas, are removed from the support surface by making use of the difference in solubility between the hardened and unhardened areas. Thus, an effect is achieved substantially equivalent to or better than that achieved with a lacquer application subsequent to development.
In attempting to incorporate such a resin into the composition, however, the compatibility between the resin and the diazo resin as well as suitable selection of a common solvent for both must be considered. Moreover, the composition must be formulated so that either the photosensitive coating on the whole is rendered insoluble upon the photo-decomposition of the photosensitive component, or a clear difference in solubility or swelling ability between the exposed and unexposed areas occurs. Further, the resin to be used as a binder must not degrade the storage stability of the resulting printing plate in which it is present together with the photosensitive diazonium compound. Certain kinds of water-soluble colloids are known to provide photoresist images of excellent quality when used as a binder for a diazo resin. However, the photosensitivity is completely lost in several days only due to dark reactions. Although various proposals have been made for a material meeting the requirement just mentioned in a number of known approaches, none of these known approaches satisfy all the requirements as to hardening efficiency, durability in printing, oleophilicity, storage stability, etc.
Japanese patent application (OPI) No. 118,802/1975 (corresponding to U.S. patent application Ser. No. 551,106, filed Feb. 20, 1975) discloses a photosensitive composition which meets almost all of these requirements, in which the composition comprises a binder polymer comprising 2 -hydroxyethyl methacrylate, acrylonitrile, methyl methacrylate and methacrylic acid and a diazonium compound. When such a photosensitive composition comprising the above-described polymer and a diazonium compound is used for a pre-sensitized lithographic printing plate, excellent properties are achieved. Unfortunately, probably because adhesion to the metal support is too strong, particularly to an anodized aluminum plate, such a photosensitive coating, when exposed through a transparent negative original after several months from production thereof have elapsed, gives rise to an imperfect removal of the non-irradiated areas upon development, thus stains or scumming in the non-irradiated areas occurs. Such a disadvantageous tendency becomes more marked when the plate has been stored under adverse storage conditions (e.g., at 45.degree. C. and 75% RH) for several days.